While trying to add Symfony 4 support to LiipThemeBundle I came across this: the bundle calls Symfony\Bundle\FrameworkBundle\Templating\Template\TemplateNameParser::parse() with a simple name (not containing a bundle): index.html. This creates a TemplateReference with no bundle, but TemplateReference::getPath() returns views/index.html.twig for the path. As with the twig engine the templates is searched in templates/ directory, this looks like a bug.